1. Field of the Invention
The present invention relates to solid oxide fuel cells.
Recently, fuel cells have been recognized as power generating equipment. Since the fuel cell is a device capable of directly converting chemical energy possessed by fuel to electrical energy and the fuel cell is free from any limitation of Carnot's cycle, the cell is an extremely promising technique in that the fuel cell essentially has a high energy conversion efficiency, a variety of fuels (naphtha, natural gas, methanol, coal reformed gas, heavy oil, etc.) may be used, the cell provokes less public nuisance, and its power generating efficiency is not influenced by the scale of the equipment.
2. Related Art Statement
Particularly, since the solid oxide fuel cell (SOFC) operates at high temperatures of 1,000.degree. C. or more, reaction on the electrode is extremely active. Thus, no catalyst of a noble metal such as expensive platinum is necessary. In addition, since the SOFC has low polarization and relatively high output voltage, its energy conversion efficiency is conspicuously higher than that of other fuel cells. Furthermore, since their constituent materials are all solid, SOFC is stable and has long use life.
FIG. 14 is a schematic sectional view illustrating an example of such an SOFC.
In FIG. 14, reference numerals 1, 4, 5 and 6 are an oxidizing gas feed pipe for the introduction of an oxidizing gas such as air, a bottom-provided porous support tube, an air electrode, and a solid electrolyte, respectively. Reference numerals 7, 8, 9 and 10 are a fuel electrode, an upper plate for holding the oxidizing gas feed pipe 1 and separating an oxidizing gas chamber 18 from an exhaust gas chamber 19, a plate having a gas outflow hole 9a an separating the exhaust gas chamber 19 from a fuel reacting chamber 20, and a bottom plate provided with fuel inflow holes 10a and adapted for holding an SOFC element 40 and separating a cell reacting chamber 20 and a fuel chamber 30, respectively.
In this state, when the oxidizing gas such as air is fed from the oxidizing chamber 18 to the oxidizing gas feed pipe 1 as shown by an arrow A, the oxidizing gas flowing out through an oxidizing gas feed opening 1a is inverted at a bottom portion 4a (arrows B), flows through a space 29 inside the bottom-provided cylindrical porous support tube 4, and is discharged out to the exhaust gas chamber 19 as shown by an arrow D. On the other hand, when a fuel gas such as H.sub.2 or CH.sub.4 flows along the outer surface of the SOFC element 40 through the fuel inflow openings 10a of the bottom plate 10, the fuel gas reacts with oxygen ions diffusing out through the solid electrolyte on the surface of the fuel electrode 7. As a result, current flows between the air electrode 5 and the fuel electrode 7, so that the SOFC can be used as a cell. Since this fuel cell is used at high temperatures such as around 1,000.degree. C., it is preferably used in the construction of FIG. 14 which needs no sealed portion.
In order to put the SOFC into practical use, it is necessary that costs are reduced, and electrical power density is increased. For this reason, it is required that the length of the SOFC element 40 is increased and that the power generation output per element is increased.
However, when the bottom-provided tubular SOFC element 40 is prolonged the temperature gradient becomes greater owing to non-uniform reactivity on the electrode in the longitudinal direction of the SOFC element 40, so that thermal strain and stress become greater to develop cracks in the SOFC element and shorten the use life thereof.
Further, the power generation amount of the SOFC is greatly influenced by the amount of oxygen permeating the bottom-provided porous support tube 4.
That is, since the concentration of oxygen is still high near the oxidizing gas feed opening 1a, the amount of oxygen ions reaching the fuel electrode 7 near there is great, so that the reacting rate between the oxygen ions and the fuel on the surface of the fuel electrode 7 is large to raise the temperature. With this increase in temperature, the reaction on the fuel electrode between the oxygen ions and the fuel gas is further activated. On the other hand, as the gas flowing out through the oxidizing gas feed opening 1a approaches the side of the gas outflow hole 9a, the concentration of oxygen in the gas decreases. Consequently, the amount of oxygen ions reaching the surface of the fuel electrode 7 near the gas outflow hole 9a decreases, so that the reaction amount between the oxygen ions and the fuel on the fuel electrode 7 is small to lower the elevation of the temperature. As a result, the reaction is further inactivated due to its lower temperature. This tendency becomes more conspicuous when the bottom-provided SOFC element becomes longer.
In addition, with recent improvements on the performance of the fuel cell, support tubes having excellent oxygen-diffusing properties have been required.
Similar problems occur in the case of the SOFC in which a fuel electrode is provided inside a solid electrolyte 6 and a fuel gas is passed through a space 29 inside a tube for the power generation, too. In such a case, since a significant amount of CO.sub.2, steam, etc. are contained in the fuel gas having its concentration reduced, these ingredients attach to the surface of the electrode to hinder the reaction. Thus, the reaction becomes less active, so that the temperature of the SOFC element becomes considerably non-uniform in the longitudinal direction.